An electrical connection to the loadboard is basically accomplished during a single insertion/compression of the device under test on to the contact pins by applying a certain amount of force. The conductive pads/leads of the device under tests generally form a layer of oxides that need to be broken through to make a successful connection and testing of the DUT. In addition to the natural oxide barriers, there are residual organic materials that remain on the surface of the conductive leads/pads. These migration of the oxides to the contact tip of the test pin acts as a thin insulating layer that makes it very difficult to achieve and maintain very low contact resistances on testing thus, negatively affects test results and ultimately reducing device yields.
Most of the time the device under test (DUT) undergoes testing with spring actuated probes or vertical probing, where the mentioned residues will remain imposed on to the contact tip of the pin resulting it to very high contact resistance leading to poor test yields. There is too the trend of testing the DUT that are plated with NiPd/NiPdAu on the contact leads/pads which are often considered as the hard contact surface and the probes that test this surface contact often experience premature wearing of the contact tip.
For devices aimed at Kelvin testing, there are known in the art cantilever type, where one of the contacts is formed as a cantilever, and sandwich types, where both contacts are sandwiched over an electrical insulator. With both these types, there is minimal wiping action on the pad/lead of the chip. With the sandwich type, the width of the contacts is very thin, thus making it structurally weak and prone to breakage. Also with the sandwich type, assembly for fine pitching is difficult.
It is highly challenging to test a multi row quad flat no lead device using normal test solutions such as cantilever pins or rigid pins contact methods. The conventional multi row QFN pads generally forms a pattern of chessboard, where the first row and second row are placed at certain distance to each other. The testing of such multi raw QFN is made possible through the “double decking” of the short pin over a tall pin by means of an interposer contact to load board.
What is needed is an improved chip testing solution that overcomes the above problems for a Kelvin testing solution as well as for multi-row chip testing.